1. Field Of The Invention
The invention relates to a data receiver comprising deriving means for deriving a detection signal from an input signal, an output of the deriving means being coupled to an input of detection means which determine a most probable sequence of data symbols on the basis of the detection signal, the symbols occurring in the detection signal with a symbol interval T, by recursively updating candidate sequences (survivors) with associated path metrics derived from branch metrics, the branch metrics being an even function of a difference value e.sub.k, present at a sampling instant kT of the detection signal and of the detection signal expected for the relevant candidate survivor, while k is an integer.
2. Description Of The Related Art
A receiver of this type is known from the journal article "On the use of decision feedback for simplifying the Viterbi detector" by J. W. M. Bergmans et al., in Philips Journal of Research, Vol. 42, No. 4, 1987.
Receivers of this type may be used, for example, for data signal transmission through the public telephone network or for reconstructing data signals from a magnetic tape or disc.
When data symbols are transmitted by means of a transmission medium or stored on a recording medium, respectively, the data symbols to be transmitted or recorded are converted to analog pulses which are applied to the transmission medium and recording medium, respectively.
Generally, the analog pulses are provided not to overlap in time. If the medium has a limited bandwidth, the pulses will start to overlap which will often lead to the fact that a detection signal received at a specific moment does not only depend on a single data symbol but also on data symbols adjacent in time. This effect is called intersymbol interference.
In addition to being caused by a limited bandwidth of the medium, intersymbol interference may also be caused by the use of a band limiting filter at a transmitter end used for giving the frequency spectrum of the transmitted or recorded analog pulses a desired shape. The presence of intersymbol interference will often lead to an enhanced bit error rate.
A possibility of reducing the enhancement of the bit error rate caused by intersymbol interference is the use of an intersymbol interference equalizer which is adaptive or not. This equalizer may have the form of a filter with a transfer function that is inverse to the transfer function of the medium. By cascading this filter with the medium, a more or less flat transfer characteristic is obtained, so that the intersymbol interference disappears. However, a problem may be that the filter considerably amplifies noise already available.
Another possibility of reducing intersymbol interference is the use of a decision feedback intersymbol interference compensator. In this compensator a compensation signal is subtracted from the received detection signal on the basis of decisions taken about data symbols that have already been received. This compensation signal is an estimate of the postcursive intersymbol interference belonging to the received symbol. This intersymbol interference occurs subsequent to the cursor (main pulse) belonging to the data symbol. Since the compensation signal is determined by a decision about the value of a received data symbol, which decision may in fact be erroneous but does not further cause any noise, there is no noise gain now.
A drawback of the use of a decision feedback intersymbol interference compensator is that precursive intersymbol interference, preceding the cursor, cannot be compensated because first the received data symbol is to be known before the compensation signal belonging to the data symbol can be generated.
An optimum receiver capable of eliminating the effect of both precursive and postcursive intersymbol interference determines all possible sequences of transmitted data symbols and the associated detection signals which would have been received if the sequence of data symbols were transmitted by the channel considered noise-free. By comparing all the detection signals thus obtained to the recent detection signal, the most probable sequence of transmitted data symbols can be determined. Such a receiver, however, requires an impracticably large computing and storage capacity.
In the receiver known from the aforementioned journal article, the most probable sequence of transmitted data symbols is determined by the detection means by recursively updating a limited number M=L.sup.N of candidate survivors in which L is the number of levels of the transmission or recording signal used and N is the impulse response length of the cascaded send filter, of the medium and of the receive filter expressed in numbers of samples. This number is necessary because the channel may have M states and the receiver is to be capable of distinguishing between these states.
Once the M candidate survivors with associated path metrics have been determined, each candidate survivor is extended upon the arrival of a next data symbol to a plurality of candidate survivors of which only the data symbols extended last differ. The path metric associated to each new candidate survivor is determined on the basis of the path metric of the candidate survivor from which the new candidate survivor is derived and the branch metric associated to this new candidate survivor.
In order to achieve that the necessary computing and storage capacity still continues to be independent of the length of the transmitted data sequence, the number of candidate survivors is reduced by removing the oldest data symbol from each candidate survivor. Such detection means are usually referred to as a Viterbi detector.
For optimum detection it is necessary that the difference values be calculated at the correct sampling instants. Since these correct sampling instants are not known to the receiver, these sampling instants are to be derived from the detection signal. Nothing about deriving the sampling instants from the detection signal can be gathered from the aforementioned journal article.